1. Field of the Invention
The present invention pertains to the filtration of water, primarily for drinking purposes.
2. Description of the Background
The importance and desirability of filtering water prior to drinking is becoming increasingly apparent. Water which is classified as potable nevertheless often contains chemicals or other substances which, while not necessarily likely to cause immediate life-threatening illness, may nevertheless be less than ideal in terms of the long-term health of the consumer and may also make the water aesthetically unpleasing.
One of the best filtering media is activated carbon, and carbon filters, for filtering air, water and other substances, are well known. One is shown in U.S. Pat. No. 4,306,971. However, a carbon filtering medium has a limited life in two respects: first, the life of the carbon, in terms of its ability to continue to adsorb contaminants and the like, is limited. Secondly, and especially where the carbon is being used to filter water, bacterial growth can occur in the carbon after a certain period of time, which also, therefore, limits the life of the carbon for purposes of filtering water which is to be consumed.
In the past, it has been known to protect carbon in filters against bacterial growth by the use of silver in association with the carbon. However, it is now known that the silver is dangerous, and should not be used in a filter for handling drinking water.
For both of these reasons, it is desirable that there be a simple and inexpensive way to time the life of a filter to alert the consumer when it is time for the filter to be changed. It is especially desirable that the timer be connected with the filter itself, and that the filter and timer be adapted for use in a filtering container so that the indication that the filter should be changed can be readily observed by a consumer without trouble and without disturbing the timer, filter and/or container.
It has been known, for other purposes, to provide timers which operate on the general principle of an indicator member moving lengthwise along a tube filled with a fluid having a specific gravity different from that of the indicator. Such devices are shown in U.S. Pat. No. 3,025,665; U.S. Pat. No. 3,553,959; U.S. Pat. No. 4,090,356; and U.S. Pat. No. 4,228,649. In some of these devices, the indicator has a lower specific gravity than that of the fluid, so that it rises in the tube, whereas in others, the indicator has a higher specific gravity so that it falls in the tube.
The timers disclosed in these prior patents are intended for other uses, typically uses over much shorter ranges of time than the life of a water filter, and under circumstances in which there is no particular problem in providing for ready visibility of the position of the indicator along the tube.
Also, these devices are intended to be reset and reused, which is typically done by inverting the tube so as to allow the indicator to begin travel from the opposite end of the tube from that employed in its last longitudinal movement.
However, a timer associated with a water filter should not be resettable, so that the consumer cannot inadvertently use the filter beyond its intended life. On the other hand, the travel of the indicator in a water filter should not begin prematurely, i.e. before the filter is put into use, so that the consumer will not replace and discard the filter too soon. Furthermore, inversion is not a desirable way even to initially set the position of an indicator in the context of a water filtration system. Therefore, there is need for a means for positively fixing such an indicator at the beginning of its intended stroke of travel, but with such a means being readily releasable when the filter is put into use.